Tunnel valley genesis

Tunnel valleys are elongate depressions with overdeepened areas along their floors cut into bedrock or unconsolidated sediment. They are frequently sinuous and form anastomosing networks, although they also exist as independent, straight valleys. Their sedimentary infill is variable but is characteristically dominated by sediment gravity flow facies and thick units of glaciofluvial sands. Till tends to be rare and, where present, occurs along valley sides. Three main theories of tunnel valley formation exist at present. The first ascribes the formation of tunnel valleys cut into unconsolidated sediment to the creep of deformable subglacial sediment into a subglacial conduit from the sides and below, followed by removal of this material through the conduit by meltwater flow. Tunnel valleys are thus created by lowering of the sediment surface on either side of the conduit. The second theory argues that tunnel valleys form during deglaciation, at or close to the ice margin, by subglacial meltwater erosion and that the valleys are time transgressive. The third theory also argues for an origin by subglacial meltwater erosion but claims that the discharges involved took the form of catastrophic channelized floods and that the tunnel valleys within anastomosing networks formed synchronously. Following an outline of tunnel valley geomorphol ogy and sedimentology, each theory is critically reviewed and conclusions are drawn. Emphasis is placed upon the importance of sedimentological and geomorphological field observations as a basis for formulating models of tunnel valley genesis.